1. Field of the Invention
The present invention relates to a process for oxidizing an .alpha.-bonding type polysaccharide, and more specifically, it relates to a process for preparing a water-soluble tricarboxypolysaccharide by oxidizing an .alpha.-bonding type polysaccharide with a high valence ruthenium oxide produced by the use of a ruthenium compound and an oxidizing agent. The water-soluble tricarboxypolysaccharide which can be obtained by the present invention can be utilized as any of a scale deposition inhibitor, a pigment dispersant, a sizing agent, a concrete blending agent, a detergent builder and the like.
2. Description of the Related Art
Heretofore, various methods for preparing a dicarboxypolysaccharide by oxidizing an .alpha.-bonding type polysaccharide are known. For example, in Japanese Patent Publication No. 1281/1974, there has been described a method for oxidizing an .alpha.-bonding type polysaccharide by the use of a combination of periodic acid and a chlorite, or a hypochlorite, and it has also been disclosed therein that a dicarboxystarch obtained by oxidizing the C.sub.2 and C.sub.3 positions of the .alpha.-bonding type polysaccharide is an excellent detergent builder.
Furthermore, Japanese Patent Application Laid-open No. 2187/1985 has described a process for preparing a dicarboxypolysaccharide from an .alpha.-bonding type polysaccharide in the presence of sodium hypochlorite, chlorine, or a combination of periodic acid and a halogen. In Japanese Patent Application Laid-open No. 175301/1992, a method for preparing a dicarboxypolysaccharide by the use of a hypochlorite and/or a hypoiodite has been disclosed.
On the other hand, with regard to a process for preparing a tricarboxypolysaccharide by oxidizing an .alpha.-bonding type polysaccharide, for example, Czechoslovakian Patent No. 235576 has described a method for preparing a tricarboxystarch from starch in the presence of a combination of a periodate and dinitrogen tetroxide. From EP-A-O 542496, there is known a process for preparing a tricarboxystarch from starch in the presence of a combination of a vanadium salt and a nitrite in a concentrated nitric acid-concentrated sulfuric acid solvent. Japanese Patent Application Laid-open No. 71601/1997 has described a process for preparing a polycarboxylic acid derived from a polysaccharide having anhydrous glucose as a constitutional unit.
However, in Czechoslovakian Patent No. 235576 mentioned above, a long process of two steps is required, and the oxidizing agent which is expensive and is available through a complicated route must be used. In addition, the obtained tricarboxystarch is not water-soluble, which is inconvenient for its use. Moreover, in the process described in EP-A-O 542496, a post-treatment of the mixed acids having the high concentrations which is to be done after reaction is not easy, and another problem such as the removal/disposal of the used metal salt are also present. For these reasons, the disclosed process is not desirable as an industrial manufacturing technique. Furthermore, these methods described above are insufficient to easily and inexpensively manufacture a water-soluble tricarboxypolysaccharide having a high carboxyl group content in a high yield which is intended by the present invention. Additionally, even by the process disclosed in Japanese Patent Application Laid-open No. 71601/1997, sufficiently satisfactory results have not been obtained so far.